Magnetic resonance imaging systems excite nuclear magnetizations of respective protons contained in a subject, who is placed in a static magnetic field, by irradiating a radiofrequency magnetic field of a specific frequency to the subject (magnetic resonance phenomenon), detect a magnetic resonance signal generated from the subject, and thus acquire physical or chemical information. Magnetic resonance imaging (hereinafter abbreviated to MRI) that has widely prevailed provides an image which reflects a density distribution of a proton contained mainly in each of water molecules in the subject. In contrast to the MRI, a method of separating one magnetic resonance signal from others in units of a molecule on the basis of a difference in a resonance frequency derived from a difference in chemical bonding of one molecule containing a proton from other various molecules each containing the proton (referred to as a chemical shift) shall be called magnetic resonance spectroscopy (hereafter abbreviated to MRS) (refer to, for example, “Journal of Magnetic Resonance” (vol. 70, pp. 488-492, 1986)).
Moreover, a method of simultaneously acquiring spectra which represent numerous areas (pixels) so as to visualize each molecule is called magnetic resonance spectroscopic imaging (hereinafter abbreviated to MRSI). The adoption of the MRIS makes it possible to visually grasp a concentration distribution of each metabolite (refer to, for example, “MRM 30” (pp. 641-645, 1993)).
Normally, the concentration of a metabolite contained in a subject is often very low. When the MRS or MRSI is performed for measurement, unless a signal of high-concentration water is suppressed, a feeble signal of a metabolite is buried in the skirt extending from the peak of the strong signal generated from water. This makes it difficult to separate or sample the metabolite signal. Consequently, in the existing MRS or MRSI, preprocessing intended to suppress the water signal is performed immediately previously of normal excitation and detection.
During the processing intended to suppress the water signal, first, a radiofrequency magnetic field is irradiated with a transmission frequency set to the frequency at the position of the peak of the water signal and an excitation frequency band narrowed to the peak width of the water signal. This is intended to excite only nuclear magnetizations contained in water molecules. Thereafter, the phases of the nuclear magnetizations contained in the excited water molecules are differentiated from one another, and a dephasing magnetic field is applied in order to nullify the sum of the magnetization vectors (pseudo saturation). While the pseudo saturation of the nuclear magnetizations in the water molecules continues, normal excitation and detection are performed in order to measure a feeble signal of a metabolite.
Moreover, since a signal of a metabolite is quite feeble, as long as measurement is performed conventionally through the MRS or MRSI, numerous averagings and measurements have to be performed in order to improve a signal-to-noise ratio (SNR) of an obtained spectrum.